Coronavirus tests are extremely sensitive. (That could be a problem, experts say.)
September 1, 2020
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"We've been using one type of data for everything, and that is just plus or minus—that's all," Michael Mina, an epidemiologist at the Harvard T.H. Chan School of Public Health, explained. "We're using that for clinical diagnostics, for public health, for policy decision-making."
But other types of tests can also provide information on how much of the virus a person is carrying, Mandavilli reports. According to Mandavilli, PCR tests look for genetic matter from the new coronavirus using amplification cycles. The fewer cycles needed to detect genetic matter from the virus, the higher a patient's viral load and the more likely that person is contagious, Mandavilli reports.
However, the number of amplification cycles that was needed to detect genetic matter from the virus, which is referred to as the cycle threshold, typically isn't included in test results sent to doctors and patients, Mandavilli reports.
Mina said that information is more telling, as it indicates a patient's viral load, and patients carrying minuscule amounts of the virus may not be contagious. Therefore, information on a patient's viral load should be used to determine the patient's next steps, Mina said.
"It's really irresponsible, I think, to forgo the recognition that this is a quantitative issue," he added.
But further complicating the matter, according to some experts, is that many commonly used tests for the novel coronavirus are too sensitive, so they generate positive results when patients are carrying low loads of the virus, Mandavilli reports.
According to Mandavilli, many coronavirus tests have fairly high cycle thresholds, with most set at 40 and some set at 37. That means a number of patients who aren't carrying much of the new coronavirus are still testing positive, even though they may not be contagious, Mandavilli reports.
Mina explained that tests with high cycle thresholds could be detecting genetic fragments of the virus, or pieces of the virus that are leftover from a previous infection that don't pose any current transmission risk.
Juliet Morrison, a virologist at the University of California-Riverside, said she believes any test with a cycle threshold over 35 is too sensitive. "I'm shocked that people would think that 40 could represent a positive," she said.
And according to Mandavilli, a review conducted by the New York Times of three sets of coronavirus testing data from Massachusetts, Nevada, and New York found that up to 90% of patients in those data sets who tested positive for the coronavirus had very low viral loads.
Ashish Jha, director of the Harvard Global Health Institute, said of the Times review's results, "I'm really shocked that it could be that high—the proportion of people with high [cycle threshold] value results." He added, "Boy, does it really change the way we need to be thinking about testing."
How should testing change, according to the experts?
Morrison said a better threshold would be 30 to 35, while Mina said he'd recommend setting the threshold to 30 or less.
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https://www.advisory.com/daily-briefing/2020/09/01/covid-tests